As the method of producing the polycrystalline silicon which is also called as the polysilicon, Siemens method is known. According to Siemens method, the silicon core wire placed inside of a bell jar type reaction container is heated to a silicon depositing temperature, then hydrogen and silane compound gas such as tricyclosilane (SiHCl3) and monosilane (SiH4) or so are supplied to deposit the polycrystalline silicon on the silicon core wire by a chemical vapor phase deposition method, thereby a highly pure polycrystalline silicon rod is obtained.
The obtained polycrystalline silicon rod is fractured and sorted into a suitable size for the device used in the subsequent steps or for the production of object to be produced in the subsequent steps, then transferred to the next step. Specifically, the polycrystalline silicon rod is crushed by a hammer made of hard metal such as tungsten carbide or so, thereby the polycrystalline silicon block material is obtained. Then, the polycrystalline silicon block material is further fractured to a desired particle size by fracture device made of hard polymer or hard metal or so, and if needed, classified to a desired size by a classifier made of same material, thereby the polycrystalline silicon fragment having a desired particle size is obtained.
The obtained polycrystalline silicon fragment is called, a dust, a powder, a chip, a nugget, and a chunk or so depending on the size thereof, however there is no strict standard for the classification. In the present invention, the fragment having the particle size of less than 500 μm is called “silicon dust”, the particle size of 500 to 1000 μm is called “polycrystalline silicon powder”; and the polycrystalline silicon block including the polycrystalline silicon fragment piece of a desired size, the polycrystalline silicon powder, and silicon dust obtained as mentioned in the above is called “polycrystalline silicon fragment”.
During the fracturing and sorting of the polycrystalline silicon rod, the rod and the fragment contact with the fracture device and sorting device, and the contamination from these devices adheres to the surface oxide layer of the surface. Also, the metal fine powder due to the abrasion in the fracture device adheres to the surface oxide layer of the fragment, and may contaminate. These are called a surface metal contamination. The surface metal contamination tends to increase as the particle size of the fragment decreases, and the contamination adheres particularly to the silicon dust and the polycrystalline silicon powder having small particle size, thereby the surface metal contamination is increased.
In order to reduce the surface metal contamination, a wet chemical treatment such as treating the polycrystalline silicon with acid or so is widely done. By such wet chemical treatment, the silicon dust or the polycrystalline silicon powder which has a small particle diameter are removed; and also the polycrystalline silicon powder having large particle diameter and the surface contamination of the polycrystalline silicon fragment are removed, thus the amount of the silicon dust and the surface metal contamination attached to the polycrystalline silicon fragment can be reduced to the order of ppbw or less. Therefore, in case extremely high purity is demanded for the growth material of the silicon single crystal, the wet chemical treatment is carried out, so that the impurity level of the polycrystalline silicon fragment can be reduced as little as possible. For example, the patent document 1 (JP Patent Application Laid OPEN No. H06-144822) discloses to form a fine particle by pulverization, and when the above mentioned polycrystalline silicon powder has the particle diameter of 1000 μm or less, the impurity of the metal powder or so caused by the abrasion of the pulverizer becomes prominent; hence it clearly asserts that the wet chemical treatment is necessary if the use for the semiconductor is considered (see [0009]). However, the wet chemical treatment is costly. Also, it is indicated that when using for the semiconductor, regarding the silicon dust and the polycrystalline silicon powder having small particle diameter as small as 1000 μm or less, a sufficient high purification may not be possible even by the above mentioned wet chemical treatment.
On the other hand, the polycrystalline silicon used for the production of the solar panel or so does not require extremely high purity as discussed in the above. Therefore, depending on the use, the cost reduction may be prioritized without increasing the purity too much as long as the amount of the silicon dust and the metal surface contamination are below acceptance level.
As the method for reducing the surface metal contamination by a relatively low cost, the patent document 2 (JP Patent Application Laid Open No. 2012-46412) proposes blowing a compressed air and dry ice or so to the polycrystalline silicon of after the fracturing and classification, thereby removing the silicon dust.
However, when compressed air is blown to the polycrystalline silicon fragment, the silicon dust can be removed by the air pressure, but at the same time, the silicon fragment having large particle diameter than the silicon dust may jump, or collide against each other, and new silicon dust or the polycrystalline silicon powder may be formed. Also, when fracturing the silicon rod, the oxide film is formed at the fracture face of the fragment. If the silicon dust and the polycrystalline silicon powder adhere to this oxide film, said oxide film of the surface and the oxide film of the fracture surface of the fragment may be integrated into one body, and the removal of these silicon dusts and the polycrystalline silicon powders may become difficult. Further, if the compressed air is blown to the stacked polycrystalline silicon of after the classification, the silicon dust and the polycrystalline silicon powder between the spaces of the stacked fragment cannot be removed sufficiently.
Further, the patent document 2 removes the silicon dust having the particle size of less than 400 μm. For the fragment having the particle size of 500 μm (0.5 mm) or more, it is recognized as the product, and will not be removed. However, the particle having the particle size of 500 μm to 1 mm or so, which is called as “polycrystalline silicon powder” in the present invention still has a prominent mixing amount of the metal impurity as shown in the patent document 1, and also when the present inventors have examined, the following described problems were found.
The polycrystalline silicon fragment is used for the production of the single crystal silicon ingot in some cases. When producing the single crystal ingot, it is necessary to smoothly carry out the filling of the polycrystalline silicon fragment to the molten container. However, if the polycrystalline silicon powder of 500 μm to 1 mm or so is included, the fluidity of the polycrystalline silicon fragment may be compromised, and the smooth introduction to the molten container may be interfered. Also, for the ingot production, the polycrystalline silicon fragment is melt in the molten container, and then pulled out (also called as Czochralski method (CZ) method). However, the powder of 500 μm to 1 mm or so may be difficult to melt, and it may become a core of the crystalline production, thus the ingot may be polycrystallized. Therefore, in order to produce the single crystal ingot, the polycrystallized ingot needs to be re-melt, and pulled out once again, which lowers the productivity.
Also, when the silicon fragments are stacked, the fine silicon dust can be removed by blowing the compressed air, but the polycrystalline silicon powder of 500 μm to 1 mm or so is difficult to be removed. If the pressure of the compressed air is too strong, then as mentioned in the above, the silicon fragment may jump, or the fragments may collide against each other, thus new silicon dust and the polycrystalline silicon powder may be generated.
The patent document 3 (JP Patent Application Laid Open No. 2009-78961) discloses that in order to prevent the dust from adhering to the polycrystalline silicon which is electrically charged, an ionized clean air is blown to the polycrystalline silicon, thereby removes the electro static.
Also, when fracturing the polycrystalline silicon rod, it is preferable to prevent the metal from adhering as much as possible to the fragment surface from the devices, and in said fracture device, it has been experimented to constitute the material of the outside force loading member against the polycrystalline silicon rod, specifically the movable teeth and stationary teeth for the jaw crusher, by a hard metal such as tungsten carbide or so (for example see JP Patent Application Laid Open No. 2004-161595); and the patent document 4 discloses (JP Patent No. 4351666) the production device to pulverize the polycrystalline silicon with the pulverizer comprising an ultrahard pulverizing apparatus, then classifying.